1. The History of Astronomy Part Three
Eratosthenes
The size of the Earth

2. Eratosthenes ( BC)
was the first person to accurately calculate the size of the Earth.
On the Summer Solstice
he used
the shadow of an obelisk to find the angle of the Sun’s rays at Alexandria (82.8°)
He knew
That at Syene, in southern Egypt the Sun shone straight down a well thus the angle there was 90º

3. The difference in the angles was about 7.2°
or 1/50th of a circle
Earth’s Circumference.
A little multiplication gave him the Circumference to be about 25,000 miles, very close to its true size.

4. Columbus and the size of the Earth
Using an incorrect number for the size of the Earth (from Strabo) of 18,000 miles…
Columbus thought the spice islands where 7000 miles closer than they were.
If it hadn’t been for the Americas Columbus and his men would have starved as some of Isabella’s advisors had warned they would!
(Isabella’s Advisors knew the Earth was round…
but they thought it was larger…
which it is!)